If you have a Mac OS X 10.3 Installation Disk, the best way is to restart from the CD, run Disk Utility from the menu, and Erase with the Option to Zero Data. Then reinstall Mac OS X 10.3. It would probably save you time, compared to having to selectively delete files and leave the software intact.
There are probably utilities to erase only the free space securely. The Disk Utility from Mac OS X 10.4 has a Erase Free Space button that is not in the 10.3.
If your iMac is one with FireWire, you could restart it in FireWire Target Disk mode. Actually, first you should delete everything you want to securely erase.
http://docs.info.apple.com/article.html?artnum=58583
Then, connect it to one of your Macs with 10.4. The iMac's hard drive should appear on the other Mac's desktop as a FireWire external drive. Run Disk Utility, select the iMac's drive, and go to the Erase tab. There should be a button there that says Erase Free Space.